1. Field of the Invention
The present invention relates to a light-emitting material (i.e., a light-emitting device material) and light-emitting device capable of converting electric energy to light which is then emitted and more particularly to a light-emitting device which can be preferably used in various arts such as display device, display, backlight, electrophotography, illuminating light source, recording light source, exposure light source, reading light source, sign, advertising display and interior. The present invention also relates to a novel light-emitting material which can be expected to find application in various arts.
2. Description of the Related Art
Today, various display devices have been under active study and development. In particular, an organic electric field light-emitting (EL) device can emit with a high luminance at a low voltage and thus has been noted as a favorable display device. For example, a light-emitting device having a vacuum-deposited thin organic layer has been known (Applied Physics Letters, vol. 51, page 913, 1987). The light-emitting device described in this reference comprises as an electron-transporting material tris(8-hydroxyquinolinate) aluminum complex (Alq) which is laminated with a positive hole-transporting material (amine compound) to exhibit drastically improved light-emitting properties as compared with the conventional single-layer type devices.
In recent years, the application of organic EL device to color display has been under active study. However, in order to develop a high performance color display, it is necessary that the properties of blue, green and red light-emitting devices be each improved.
As a means for improving the properties of light-emitting devices there has been reported a green light-emitting device utilizing the emission of light from orthometalated iridium complex (Ir(ppy)3: Tris-Ortho-Metalated Complex of Iridium (III) with 2-Phenylpyridine) (Applied Physics Letters 75, 4 (1999)). The foregoing device can attain an external quantum yield of 8%, which is higher than the limit of the external quantum yield of the conventional light-emitting devices, i.e., 5%. However, since the foregoing light-emitting device is limited to green light-emitting device, the range within it can be applied as a display is narrow. It has thus been desired to develop light-emitting materials capable of emitting light having other colors.
Noting a red light-emitting device, many light-emitting devices comprising DCM (4-(Dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran) and its analogy have been reported. No devices having an external quantum efficiency of more than 5% have been reported. If the external quantum efficiency of 5%, which is considered to be the limit of the external efficiency of the conventional red light-emitting device, can be surpassed, the development of high efficiency organic EL devices capable of emitting light having various colors can make a great progress. It has thus been desired to develop such high efficiency organic EL devices.
On the other hand, an organic light-emitting device which, can attain light emission with a high luminance is one having a laminate of vacuum-deposited organic material layers. The preparation of such a device is preferably accomplished by a coating method from the standpoint of simplification of production procedure, workability, area attained, etc. However, the device prepared by the conventional coating method is inferior to that prepared by vacuum evaporation method particularly in light-emitting efficiency. It has thus been desired to develop a novel light-emitting material.
In recent years, various materials having fluorescence have been used in various arts such as filter dye, color conversion filter, dye for photographic material, sensitizing dye, dye for dyeing pulp, laser dye, fluorescent medicine for medical diagnosis and organic light-emitting material. Thus, there is a growing demand for such a material. New light-emitting materials have been desired.